This invention is directed to a method for simultaneously removing mercury and water from fluids by contacting the fluids with zeolite A activated by silver or gold. The invention further relates to silver- or gold-activated zeolite A and to methods for removing mercury and water from fluids using such activated zeolite A.
It is often desirable to remove water from fluids, such as from hydrocarbon fluids, an example being natural gas. Water can be effectively removed from such fluids by means of molecular sieves, particularly the synthetic crystalline zeolite known as zeolite A.
Zeolite A contains cavities formed by sodalite cages stacked in simple cubic form. The sodalite cages are made up of truncated octahedra having a silica or alumina tetrahedron at each point. The cavities are surrounded by eight oxygen atoms, and are partially blocked by cations that balance the charge on the oxygen atoms. In zeolite A, each alumina moiety is balanced by two positive charges. If the cation is sodium, the cavity is reduced to about 4.2 angstroms in diameter. If the cation is potassium, the cavity is reduced to about 3 angstroms in diameter. If the cation is calcium, the cavity is reduced to about 5 angstroms in diameter.
Zeolite A having sodium, potassium, and calcium ions are known as zeolite 4A, zeolite 3A and zeolite 5A, respectively. The pore diameters of zeolite A make them especially suitable as drying agents, since the pores are large enough to accommodate water molecules, but not most other molecules found in nature Zeolite A is further described in U.S. Pat. Nos. 2,882,243; 2,982,612; and 3,650,687.
When a zeolite used for drying fluids becomes saturated with water, it must be regenerated, which is often accomplished by heating with flowing hot gas. Zeolite 4A is the most commonly used molecular sieve for this purpose.
It is also important to remove mercury from fluids. For example, natural gas may contain as much as 250 ppb (micrograms/m.sup.3) mercury. Following the drying procedure described above, the natural gas is, in many commercial liquefaction operations, transported to aluminum heat exchangers. Mercury present in the natural gas causes corrosion of the aluminum, and must be removed.
Methods for removing mercury from fluids such as natural gas are available. For example, U.S. Pat. Nos. 4,101,631 and 4,474,896 describe the removal of mercury from gas streams by means of sulphur or sulphur compounds on supports such as zeolites and activated carbon. Such methods are capable of reducing the level of mercury to about 0.1 ppb. Even this level of mercury in a stream, however, can injure aluminum heat exchangers.
There is, therefore, a need to reduce the level of mercury in fluids to below 0.01 ppb or less. In order to be commercially feasible, the method should be inexpensive as well as efficient. Since water must often also be removed from hydrocarbon fluids, it would be especially desirable to be able to remove mercury and water simultaneously and repeatedly from a fluid with the same agent, so that, following contact with the agent, the level of mercury is less than 0.01 ppb and the level of water is less than 1 ppm. A composition of matter capable of simultaneously removing mercury and water from a fluid must be able to be regenerated many times simultaneously for both of these purposes in order to be considered efficient enough to be commercially feasible.
It is a principle object of the present invention to provide a molecular sieve capable of simultaneously removing water and mercury from fluids and of being able to be simultaneously regenerated many times for both these purposes. Further objectives of the present invention are to provide a process for making such a molecular sieve and a method for using it to remove water and mercury simultaneously from fluids.
These and other objectives as will be apparent to those of ordinary skill in the art have been met by providing a regenerable molecular sieve comprising 0.001-15% elemental silver or gold in or on zeolite A. The molecular sieve is capable of simultaneously removing mercury and water from a fluid. A method for impregnating zeolite A with elemental silver or gold comprising treating the zeolite A with a solution of a silver or gold salt of a carboxylic acid and heating the impregnated zeolite A at a temperature sufficient to decompose the salt is also provided. Finally, a method for simultaneously and repeatedly removing mercury and water from a hydrocarbon fluid by contacting the fluid with an effective amount of a molecular sieve comprising zeolite A and 0.001-15% elemental silver or gold and regenerating the zeolite A at elevated temperatures is also provided. Where silver is discussed as the metal for removing mercury, it is to be understood that gold may generally be substituted for silver.